This invention relates to an optical switch and a wavelength converter used for optical communications or optical information processing, and more particularly to, an optical switch and a wavelength converter used for TDM (time division multiplexing) optical communications.
A so-called all-optional switch and a wavelength converter are known generally. Using these optical switch and wavelength converter, very fast optical communications can be realized.
A known example of such an optical switch is a symmetrical Mach-Zehnder type all-optical switch (hereinafter referred to as xe2x80x98prior art 1xe2x80x99) disclosed in Japanese patent application laid-open No.7-20510 (1995) and Japanese Journal of Applied Physics, vol.32, pp.1746-1749, 1993. Although this optical switch is intended as a TDM demultiplexer, it can also generate optical pulse with a wavelength different from that of input pulse (Applied Physics Letters, vol.65, pp.283-285, 1994). Therefore, it can also function as a wavelength converter.
A further known example is a high-stability polarization separation type all-optical switch (hereinafter referred to as xe2x80x98prior art 2xe2x80x99), which is a modification of prior art 1 above, disclosed in Japanese patent application laid-open No.8-179385 (1996) and Applied Physics Letters, vol.67, pp.3709-3711, 1995. Also, another example of a polarization separation type all-optical switch, which operates in mechanism similar to that of prior art 2, is reported in IEEE Photonics Technology Letters, vol.8, pp.1695-1697, 1996. These polarization separation type optical switches can also function as a wavelength converter.
On the other hand, an all-optical switch (hereinafter referred to as xe2x80x98prior art 3xe2x80x99) that a Mach-Zehnder type interferometer is replaced by a combination of Sagnac type interferometer and semiconductor optical amplifier is reported in Electronics Letters, vol.30, pp.339-341, 1994. In this prior art 3, the operational principle is analogous to those in prior arts 1 and 2 and the operation can be conducted as fast as that in prior arts 1 and 2.
Further, a DISC type wavelength converter (hereinafter referred to as xe2x80x98prior art 4xe2x80x99) that the structure of the all-optical switch in prior art 2 is simplified is disclosed in Japanese patent application No.09-111633 (1997) and IEEE Photonics Technology Letters, vol.10, pp.346-348, 1998.
In prior arts 1 to 3, the all-optical switches extract optical signal from Return-to-Zero (RZ) optical signal sequence at intervals of certain time (TDM demultiplexer). The timing of extraction is controlled by control optical pulse that is input to the all-optical switch, with RZ optical signal sequence. By these all-optical switches, ultra-high-speed RZ optical signal sequence with a signal interval much shorter than carrier lifetime in the non-linear semiconductor waveguide or semiconductor optical amplifier can be demultiplexed. Meanwhile, the carrier lifetime in semiconductor is as long as 100 ps to 10 ns.
In converting the wavelength of RZ optical signal by using the all-optical switch described in prior arts 1 to 3, RZ optical signal sequence with a wavelength of xcex1 is input to the input port of control optical pulse of the all-optical switch, and continuous light with a wavelength of xcex2 is input to the input port of optical signal. Thereby, according to the existence of input RZ optical signal pulse, the all-optical switch opens and then shuts automatically after a given time. Thus, according to the existence of input RZ optical signal pulse, continuous light with a wavelength of xcex2 turns on and then turns off automatically after a given time, being output as RZ optical signal with a wavelength of xcex2 (these operations are explained in detail later). Meanwhile, the device in prior art 4 functions only an a wavelength converter of RZ optical signal. In using them as a wavelength converter, they can output optical pulse shorter than the carrier lifetime in the non-linear semiconductor waveguide or semiconductor optical amplifier.
Here, as one example of the conventional all-optical switches, the all-optical switch in prior art 1 will be explained referring to the drawings.
Referring to FIG. 1, the all-optical switch is provided with semiconductor waveguides 10, 11, a first input port 12 to which control optical pulse is input, a second input port to which signal optical pulse is input, and signal output ports 22, 23.
Control optical pulse (wavelength xcex1) input to the input port 12 is divided into 50:50, which correspond to first and second control optical pulses, at a branch point 13. The first control optical pulse is led through a coupling point 16 to a semiconductor waveguide 10. On the other hand, the second control optical pulse is led through a coupling point 17 to a semiconductor waveguide 11. Here, the optical path length from the branch point 13 to the semiconductor waveguide 11 is longer than the optical path length from the branch point 13 to the semiconductor waveguide 10. Therefore, the time when the second control optical pulse reaches the semiconductor waveguide 11 is later than the time when the first control optical pulse reaches the semiconductor waveguide 10 (here, the delay time is represented by xcex94t).
When the semiconductor waveguides 10, 11 receive first and second control optical pulses, respectively, the refractive index of the semiconductor waveguides 10, 11 changes transitionally (so-called non-linear change of refractive index occurs). Such non-linear change of refractive index occurs due to the change of carrier density inside the semiconductor waveguide. Namely, the refractive index reduces as the carrier density increases, and the refractive index increases as the carrier density reduces (band filling effect).
When the semiconductor waveguides 10, 11 are semiconductor optical amplifiers, the refractive index increases for a certain period and then recovers. The period when the refractive index increases is nearly equal to the pulse width of control optical pulse. On the other hand, the time constant in recovery of refractive index is equal to the carrier lifetime in the semiconductor optical amplifier.
When the semiconductor waveguides are absorption-type semiconductor waveguides, the refractive index reduces for a certain period and then recovers. The period when the refractive index reduces is nearly equal to the pulse width of control optical pulse, and the time constant in recovery of refractive index is equal to the carrier lifetime in the semiconductor waveguide.
Signal optical pulse (wavelength xcex2) input to the input port 18 is divided into 50:50, which correspond to first and second signal optical pulses, at a branch point 19. The first signal optical pulse is led through the coupling point 16 and the semiconductor waveguide 10 to a coupling point 20. On the other hand, the second signal optical pulse is led through the coupling point 17, the semiconductor waveguide 11 and a phase adjuster 26 to the coupling point 20.
The first and second signal optical pulses are coupled at the coupling point 20, where interference will occur. Namely, interference light occurs. This interference light is divided into 50:50, which correspond to first and second interference lights, at a branch point 21. The first interference light is led though a wavelength filter 24 to the output port 22, and the second interference light is led through a wavelength filter 25 to the output port 23.
Meanwhile, the optical path extending from the branch point 19 through the semiconductor waveguide 10 to the coupling point 20, and the optical path extending from the branch point 19 through the semiconductor waveguide 11 and the phase adjuster 26 to the coupling point 20 compose a so-called Mach-Zehnder type interferometer. Here, the optical paths are adjusted so that the optical path length of the arm extending from the branch point 19 through the semiconductor waveguide 10 to the coupling point 20 is equal to the optical path length of the arm extending from the branch point 19 through the branch point 17, the semiconductor waveguide 11 and the phase adjuster 26 to the coupling point 20.
As described earlier, the refractive index of the semiconductor waveguides 10, 11 changes due to the control optical pulse, therefore the phase of signal optical pulse to pass through the semiconductor waveguides 10, 11 changes transitionally (so-called non-linear phase shift). Here, FIG. 2A shows an example of phase shift of signal optical pulse. In FIG. 2A, the width of control optical pulse is 2 ps, xcex94t is 25 ps, the interval of switch operation time (interval of control optical pulse) is 1 ns, and the carrier lifetime is 10 ns.
Here, provided that electric field of input signal optical pulse is EIN(t), electric field of signal optical pulse (component A, first signal optical pulse) transmitted through the semiconductor waveguide 10 is EA(t), and electric field of signal optical pulse (component B, second signal optical pulse) transmitted through the semiconductor waveguide 11 is EB(t), equations (1) and (2) below are given.                                           E            A                    ⁡                      (            t            )                          =                              1            2                    ⁢                                                    E                IN                            ⁡                              (                t                )                                      ·                          exp              ⁡                              [                                                      ⅈΦ                    A                                    ⁡                                      (                    t                    )                                                  ]                                                                        (        1        )                                                      E            B                    ⁡                      (            t            )                          =                              1            2                    ⁢                                                    E                IN                            ⁡                              (                t                )                                      ·                          exp              ⁡                              [                                                      ⅈΦ                    B                                    ⁡                                      (                    t                    )                                                  ]                                                                        (        2        )            
In FIG. 2A, the solid curve indicates the phase change "PHgr"A(t) of component A, and the dotted curve indicates the phase change "PHgr"B(t) of component B (operational example 1). Meanwhile, the semiconductor waveguide is of a semiconductor optical amplifier.
When the refractive index of the semiconductor optical amplifier changes by arrival of one control optical pulse, the phase of the signal optical pulse increases. The amount of phase increase of component B is equal to the amount of phase increase of component A. When the refractive index of the semiconductor optical amplifier recovers, the phase of signal light also recovers. The refractive index change of the semiconductor optical amplifier 11 is xcex94t later than that of the semiconductor optical amplifier 10. Therefore, the phase change of second signal optical pulse (component B) is xcex94t later than that of the phase change of second signal optical pulse (component A). Accordingly, equation (3) below is established.
"PHgr"A(t)="PHgr"B(t+xcex94t)+"PHgr"bxe2x80x83xe2x80x83(3)
Meanwhile, the amount of non-linear phase shift of the semiconductor waveguide 11 is made equal to that of the semiconductor waveguide 10. Here, the phase bias "PHgr"b is adjusted by the phase adjuster 26.
After components A, B are coupled at the coupling point 20 to generate interference light, electric field of interference light (component P, first interference light) reaching the output port 22 is given by:
EP(t)=EA(t)+EB(t)xe2x80x83xe2x80x83(4)
In contrast to this, electric field of interference light (component Q, second interference light) reaching the output port 23 is given by:                                                                                           E                  Q                                ⁡                                  (                  t                  )                                            =                                                                    E                    A                                    ⁡                                      (                    t                    )                                                  +                                                                            E                      B                                        ⁡                                          (                      t                      )                                                        ·                                      exp                    ⁡                                          [                      ⅈπ                      ]                                                                                                                                              =                                                                    E                    A                                    ⁡                                      (                    t                    )                                                  -                                                      E                    B                                    ⁡                                      (                    t                    )                                                                                                          (        5        )            
Which is the compensation component of component P.
From equations (1) to (5), equation (6) below is obtained.                                                                                           E                  F                                ⁡                                  (                  t                  )                                            =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                                                            E                      IN                                        ⁡                                          (                      t                      )                                                        ·                                      (                                                                  exp                        ⁡                                                  [                                                                                    ⅈΦ                              A                                                        ⁡                                                          (                              t                              )                                                                                ]                                                                    +                                              exp                        ⁡                                                  [                                                                                    ⅈΦ                              B                                                        ⁡                                                          (                              t                              )                                                                                ]                                                                                      )                                                                                                                          =                              xe2x80x83                            ⁢                                                1                  2                                ⁢                                                                            E                      IN                                        ⁡                                          (                      t                      )                                                        ·                  exp                                ⁢                                                      ⌊                                          ⅈ                      ⁢                                                                                                                                  Φ                              A                                                        ⁡                                                          (                              t                              )                                                                                +                                                                                    Φ                              B                                                        ⁡                                                          (                              t                              )                                                                                                      2                                                              ⌋                                    ·                                                                                                                        xe2x80x83                            ⁢                              (                                                      exp                    ⁢                                          ⌊                                              ⅈ                        ⁢                                                                                                                                            Φ                                A                                                            ⁡                                                              (                                t                                )                                                                                      -                                                                                          Φ                                B                                                            ⁡                                                              (                                t                                )                                                                                                              2                                                                    ⌋                                                        +                                      exp                    ⁢                                          ⌊                                                                        -                          ⅈ                                                ⁢                                                                                                                                            Φ                                A                                                            ⁡                                                              (                                t                                )                                                                                      -                                                                                          Φ                                B                                                            ⁡                                                              (                                t                                )                                                                                                              2                                                                    ⌋                                                                      )                                                                                        =                              xe2x80x83                            ⁢                                                                                          E                      IN                                        ⁡                                          (                      t                      )                                                        ·                  exp                                ⁢                                                      ⌊                                          ⅈ                      ⁢                                                                                                                                  Φ                              A                                                        ⁡                                                          (                              t                              )                                                                                +                                                                                    Φ                              B                                                        ⁡                                                          (                              t                              )                                                                                                      2                                                              ⌋                                    ·                  cos                                ⁢                                                                                                    Φ                        A                                            ⁡                                              (                        t                        )                                                              -                                                                  Φ                        B                                            ⁡                                              (                        t                        )                                                                              2                                                                                        (        6        )            
Therefore, the complex transmittance to signal light of all-optical switch is given by:                                                                         T                ⁡                                  (                  t                  )                                            ≡                              xe2x80x83                            ⁢                                                                    E                    F                                    ⁡                                      (                    t                    )                                                                                        E                    IN                                    ⁡                                      (                    t                    )                                                                                                                          =                              xe2x80x83                            ⁢                              exp                ⁢                                                      ⌊                                          ⅈ                      ⁢                                                                                                                                  Φ                              A                                                        ⁡                                                          (                              t                              )                                                                                +                                                                                    Φ                              B                                                        ⁡                                                          (                              t                              )                                                                                                      2                                                              ⌋                                    ·                  cos                                ⁢                                                                                                    Φ                        A                                            ⁡                                              (                        t                        )                                                              -                                                                  Φ                        B                                            ⁡                                              (                        t                        )                                                                              2                                                                                        (        7        )            
The intensity transmissivity is given by:                                           "LeftBracketingBar"                          T              ⁡                              (                t                )                                      "RightBracketingBar"                    2                ≡                              cos            2                    ⁢                                                                      Φ                  A                                ⁡                                  (                  t                  )                                            -                                                Φ                  B                                ⁡                                  (                  t                  )                                                      2                                              (        8        )            
In the example shown, the phase bias "PHgr"b (FIG. 2B) is adjusted to be:
xe2x80x83xcfx86b=xcfx80xe2x80x83xe2x80x83(9)
The phase difference xcfx86A(t)xe2x88x92xcfx86B(t) is shown in FIG. 2B and the signal intensity transmittance |T(t)|2 is shown in FIG. 2C. Before component A incurs non-linear phase shift (t less than t0=approximately 0 ps) and after component B incurs non-linear phase shift (t less than t0+xcex94t=approximately 25 ps), next equations are given.
"PHgr"A(t)xe2x88x92"PHgr"Bt="PHgr"b=xcfx80xe2x80x83xe2x80x83(10)
                                                        "LeftBracketingBar"                              T                ⁡                                  (                  t                  )                                            "RightBracketingBar"                        2                    ≡                                    cos              2                        ⁢                                                                                Φ                    A                                    ⁡                                      (                    t                    )                                                  -                                                      Φ                    B                                    ⁡                                      (                    t                    )                                                              2                                      =        0                            (        11        )            
Therefore, the transmittance of all-optical switch takes a finite value, which is not zero, only while t0 less than t less than t0+xcex94t is satisfied (FIG. 2C).
The condition that the all-optical switch extracts signal optical pulse from signal optical pulse sequence is illustrated in FIGS. 3A to 3E. The time when control optical pulse enters to the all-optical switch is t1xe2x88x92xcex94t/2 (FIG. 3A). The all-optical switch transmits signal optical pulse only between t1xe2x88x92xcex94t/2 and t1+xcex94t/2 (FIG. 3B). Meanwhile, the pulse width of signal optical pulse is shorter than xcex94t. Thus, the all-optical switch transmits only signal optical pulse (FIG. 3D) at time t1 of signal optical pulses at tN (N=xe2x88x921, 0, 1, 2, . . . ) (FIG. 3C), outputting it from the output port 22. On the other hand, signal optical pulse sequence (FIG. 3E) that only the signal optical pulse at time t1 of input signal optical pulses is removed is output from the output port 23.
Meanwhile, in using a semiconductor optical amplifier as the semiconductor waveguide, it is necessary to select such a wavelength of control optical pulse that control optical pulse can be amplified efficiently by the semiconductor optical amplifier. The control optical pulse amplified by the semiconductor optical amplifier is removed by the filters 24, 25. The intensity of signal optical pulse and the bandgap of active layer in the semiconductor optical amplifier need to be selected so that the carrier number change in the semiconductor optical amplifier by signal light passing through the semiconductor optical amplifier is negligibly smaller than the carrier number change by control optical pulse.
On the other hand, in using an absorption type semiconductor waveguide as the semiconductor waveguide, it is necessary to select such a wavelength of control optical pulse that control optical pulse can be absorbed efficiently by the absorption type semiconductor waveguide. Also, the bandgap of active layer of the semiconductor waveguide needs to be selected so that signal light passing through the semiconductor waveguide is not absorbed by the absorption type semiconductor waveguide. Meanwhile, in using the absorption type semiconductor waveguide, the sign of non-linear phase shift in FIG. 2A is reverse to that in using the semiconductor optical amplifier.
However, in the all-optical switch in FIG. 1, when the interval of operation time in the all-optical switch is short, i.e., when the interval of control optical pulse is short, the extinction ratio lowers. Here, an example of operation of all-optical switch that extracts 10 Gbps signal optical pulse sequence from 60 Gbps signal optical pulse sequence is shown in FIGS. 4A to 4C (operational example 2). Here, the width of control optical pulse is 2 ps, xcex94t is 16.6 ps, the interval of switch operation time (interval of control optical pulse) in 100 ps, and the carrier lifetime is 500 ns. Meanwhile, xcex94t is set to match with the interval of 60 Gbps signals and the interval of switch operation time is set to match with the interval of 10 Gbps signals.
As shown in the operational example 2, when the interval of operation time is short, equations 10 and 11 become inapplicable. Namely, before component A incurs non-linear phase shift (t less than t0) and after component B incurs non-linear phase shift (t less than t0+xcex94t), the phase difference xcfx86A(t)xe2x88x92xcfx86B(t) shifts from xcfx80 (FIG. 4B). Therefore, even when t less than t0 and t less than t0+xcex94t, the transmittance to signal optical pulse does not lower to zero (FIG. 4C).
The condition that the all-optical switch in the operational example 2 extracts signal optical pulse from signal optical pulse sequence is illustrated in FIGS. 5A to 5E. In this example, since the transmittance to signal optical pulse does not lower to zero even when t less than t0 and t less than t0+xcex94t (FIG. 5B), unnecessary signal optical pulse sequence leaks out of the output port 22 (FIG. 5D) and the output port (FIG. 5E) (FIG. 5D indicates the case for the output port 22 and FIG. 5E indicates the case for the output port 23). Thus, the conventional all-optical switch incurs deterioration in extinction ratio.
Here, FIGS. 6A to 6C shows an operational example (operational example 3) that 2 Gbps signal optical pulse sequence is extracted from 40 Gbps signal optical pulse sequence. Here, the width of control optical pulse is 2 ps, xcex94t is 25 ps, the interval of switch operation time (interval of control optical pulse) is 500 ps, and the carrier lifetime is 75 ns.
As shown in the operational example 3, when the carrier lifetime is short, equations 10 and 11 become inapplicable. Namely, after component B incurs non-linear phase shift (t less than t0+xcex94t), the phase difference xcfx86A(t)xe2x88x92xcfx86B(t) shifts from xcfx80 (FIG. 6B). Therefore, even when t less than t0+xcex94t, the transmittance to signal optical pulse does not lower to zero (FIG. 6C).
Thus, even when extracting signal optical pulse sequence using the all-optical switch in the operational example 3, deterioration in extinction ratio occurs like the case in FIGS. 5A to 5E.
As seen from the explanation above, in using the all-optical switches in prior arts 1 to 3 an a TDM demultiplexer, when the interval of switch operation time and the carrier lifetime are sufficiently longer than the width of switch-on time, the extinction ratio is little deteriorated. However, when the interval of switch operation time (or the carrier lifetime) comes close to the width of switch-on time, the extinction ratio deteriorates gradually.
On the other hand, in using the all-optical switches in prior arts 1 to 3 as a wavelength converter and in case of the wavelength converter in prior art 4, when the interval of optical signal and the carrier lifetime are sufficiently longer than the width of switch-on time, the extinction ratio is little deteriorated. However, when the interval of optical signal (or the carrier lifetime) comes close to the width of optical signal pulse, the extinction ratio deteriorates gradually.
Thus, when the interval of switch operation time of all-optical switch (or the carrier lifetime in semiconductor waveguide composing part of the all-optical switch) is short, there is the problem that the extinction ratio of all-optical switch deteriorates. Also, when wavelength conversion is conducted using the all-optical switch, there is a problem that the extinction ratio of output optical signal deteriorates.
Further, in prior arts 1 to 4, the all-optical switch is adjusted optimum when it stops, and to adjust the all-optical switch in operation is not considered. Namely, when it continues operating for a long time such as tens to multi-thousand hours, due to a slight change of refractive index of optical waveguide in the all-optical switch, the optimized operation conditions may drift. Therefore, it is necessary to retain the optimized operation conditions of all-optical switch in operation.
Accordingly, it is an object of the invention to provide an all-optical switch or a wavelength converter that the extinction ratio can be improved.
It is a further object of the invention to provide an all-optical switch or a wavelength converter that the limit of switch operation repetition frequency can be improved.
It is a still further object of the invention to provide an all-optical switch or a wavelength converter that the optimum operational state can be kept.
According to the invention, an all-optical switch for outputting output light by transmitting/interrupting second input light with a second wavelength synchronizing with first input light with a first wavelength, comprises:
an optical interferometer to which the first and second input lights are input and which is composed of a non-linear semiconductor waveguide;
a supervisory laser light source which supplies supervisory light with a third wavelength to the optical interferometer; and
a control means to which supervisory light output from the optical interferometer is input as output supervisory light and which controls the phase bias of the interferometer according to the output supervisory light.
According to another aspect of the invention, an all-optical switch for outputting output light by transmitting/interrupting second input light with a second wavelength synchronizing with first input light with a first wavelength, comprises:
an adjusting means which adjusts the optical intensity of the first input light to give adjusted input light;
an optical interferometer to which the first and second input lights are input and which is composed of a non-linear semiconductor waveguide;
a supervisory laser light source which supplies supervisory light with a third wavelength to the optical interferometer; and
a control means to which supervisory light output from the optical interferometer is input as output supervisory light and which controls the adjusting means according to the output supervisory light to adjust the optical intensity of the first input light.
According to another aspect of the invention, a wavelength converter for converting input light with a first wavelength into output light with a second wavelength, comprises:
an optical interferometer to which the input light and supervisory light with the second wavelength are input and which is composed of a non-linear semiconductor waveguide; and
a control means to which supervisory light output from the optical interferometer is input as output supervisory light and which controls the phase bias of the interferometer according to the output supervisory light.
According to another aspect of the invention, a wavelength converter for converting input light with a first wavelength into output light with a second wavelength, comprises:
an adjusting means which adjusts the optical intensity of the input light to give adjusted input light;
an optical interferometer to which the input light and supervisory light with the second wavelength are input and which is composed of a non-linear semiconductor waveguide; and
a control means to which supervisory light output from the optical interferometer is input as output supervisory light and which controls the adjusting means according to the output supervisory light to adjust the optical intensity of the input light.